While modern industrial techniques have made significant progress towards reducing artificial air pollution, the mounting evidence of a definite connection between air pollution and a number of serious diseases has resulted in increased public concern with the problem. In addition to artificial and man-made pollution, the air in many parts of the world is often contaminated with pollen and other naturally occuring particulate pollutants. For many people, these particles cause a variety of allergic reactions ranging from discomfort to interference with breathing and other vital functions.
Accordingly, a great deal of work has been expended in developing practical systems for removing particulate pollutants from the air. The most efficient of these systems generally have two separate stages. The first stage consists of an emitter which charges the particles to be removed, while the second stage comprises means for accumulating the charged particles.
In emitter/accumulator systems, the emitter is made of a conductive material with a large surface area, such as steel wool, or any other suitable structure. Such structures must also offer a relatively low resistance to the passage of air. During operation, the emitter is connected to the negative side of a high-voltage source. The first step in the process of filtering is driving the air to be filtered through the emitter with a fan. This has the result of putting a negative charge on the O.sub.2 molecules in the air. These O.sub.2 molecules are, in turn, attracted to particles in the air, to which they adhere. The particles leaving the emitter thus have a net negative charge.
After passing through the emitter, the air with the charged particles entrained in it is passed through the actual filter stage where the charged particles are removed. The filter usually has a plurality of passages through which the air is passed, and structure for providing an electric field which extends transverse to the direction of air flow through the passages. Typically, the passages are defined between a pair of metal elements which are insulated from each other and connected to two opposite poles of the high-voltage power supply which drives the emitter. Thus, as the air is driven through the passages by the action of the fan, negatively charged particles entrained in the air tend to move toward the positive plate where they accumulate.
After the filter has been in operation for a period of time, it tends to become filled with accumulated particles. It then becomes necessary to either clean or replace the filter. Because the construction of such structures is relatively expensive, replacement is not economical and the filters are usually cleaned. This poses some practical problems inasmuch as the task of cleaning with water and chemicals requires some time and, because of the inconvenience involved, it is often neglected. Other problems with such systems include the high cost of high-voltage generating equipment and the attendant danger of electrical shock. Nevertheless, because these systems have efficiencies in the order of 95%, they are in widespread use.
Alternative structures also find very wide employment. One of the most commonly used systems is the passive fiberglass filter. These filters are made of a relatively dense mass of fiberglass or other fibrous material which is held within a cardboard support member. Particles are removed from the air by simply driving the air through the filter with a fan. The filter presents a tortuous path to the particles, which are caught and retained in the fiberglass. When these filters lose their effectiveness, they are discarded because of their low cost and replaced with a fresh filter. Still another advantage is the elimination of the need for high-voltage generation equipment.
The simplicity of its design renders the passive filter suitable for such widely diverse applications as home air conditioners and automobile air filters. However, passive filters usually have an efficiency of only three to five percent. Moreover, if one tries to improve on this efficiency, by increasing the density or thickness of the filter, the viscous resistance of the filter rises dramatically, causing the rate of air flow through the filter to drop below practical minimums.
As an alternative, it has also been noted that the second stage of the emitter/accumulator filtering system has an efficiency of about 85% when it is used without an emitter. This is due to the natural charge on particles in the air. However, this, as far as is known, has not been pursued as a practical alternative until very recently because of the fact that a high-voltage source is still needed and it makes sense to add the emitter for the gain in efficiency.
Recently, C. G. Kalt has shown that practical air filters with efficiencies comparable to that of the second stage of emitter/accumulator filters can be achieved when driving an accumulator with a low-voltage source, provided that the construction of the filter is modified. Specifically, Kalt's filters comprise a plurality of electrically conductive filter elements which are made of mylar having a deposit of aluminum on one or both of their surfaces. The mylar sheets are exceedingly thin, typically a thousandth of an inch in thickness with a millionth of an inch of metal deposited on them. These metalized mylar elements are arranged to form a relatively large number of narrow and relatively long passages. Although it is necessary to use a great many filter elements to make such a large number of passages, the thinness of the mylar, nevertheless, results in a very low air resistance. Moreover, because the passages are narrow, the voltage may be reduced to a value in the range of several hundred volts while still retaining effectiveness. Although particles passing through the filter are not accelerated very quickly toward the sides of the passages, the passages are narrow and a significant number accumulate on the surfaces of the passages. Moreover, the mylar is inexpensive and, it is contemplated that the filters would be discarded after use.
The present invention provides a method for making filters from metalized mylar or any other suitable thin conductive material. The method for making the filters and the machinery involved is economical, simple, and reliable. An additional advantage is that in accordance with the preferred embodiment, the final product may be circular in shape which results in an efficient use of material because the useful area of many planar air filters matches the circular shape of the fan which blows air through them.